(a) Field of the Invention
The present invention relates to a reflection type liquid crystal display device (reflection LCD device) having an improved image quality and, more particularly, a reflection LCD device which cancels coloring defects occurring depending on the observing azimuth and due to a higher visual angle.
(b) Description of the Related Art
The reflection LCD device has no backlight on the rear side of the LCD panel and is widely used in a portable device driven by a battery, such as a cellular phone, due to its advantage of lower power dissipation. Most of the up-to-date reflection LCD devices operate in a twisted nematic mode (TN-mode) among a variety of operating modes used heretofore.
A variety of types for the TN-mode LCD devices have been developed and used. Examples of such TN-mode LCD devices include one having a pair of polarizing plates provided on a pair of LCD panels opposing each other, another called normally-black LCD device which includes a reflection electrode in addition to the pair of polarizing plates, and another called normally-white LCD device which includes a reflection electrode and a polarizing plate.
In general, normally-white reflection LCD device is superior to the normally-black reflection LCD device because the dependency of the reflectivity upon the cell gap and the wavelength is lower during displaying a dark state in the normally-white reflection LCD device.
The present inventor disclosed a normally-white reflection LCD device in JP Patent No. 3095005 (JP-A-2000-171788). FIG. 1 shows the layer structure of the conventional reflection LCD device disclosed in the publication, which includes a front panel 10 and a rear panel 20 opposing each other to sandwich therebetween a LC layer 30.
The front panel 10 includes a front substrate 11; a transparent electrode 12 and an orientation film 13 consecutively formed on the rear surface of the front substrate 11 which opposes the LC layer 30; and a stacked xcex/4 (xc2xc-wavelength) plate 14 and a polarizing plate 16 consecutively formed on the front surface of the front substrate 10. The stacked xcex/4 plate 14 includes a xcex/4 phase shifter (phase shifter film) 14A and a xcex/2 phase shifter 14B.
The rear panel 20 includes a rear substrate 21, and a reflection electrode 22 and an orientation film 23 consecutively formed on the front surface of the rear substrate 21, which opposes the LCD layer 30.
The stacked xcex/4 plate 14 is a birefringent plate having a thickness determined to provide a pair of light components of a linearly polarized light with an optical path difference (retardation) of xc2xc of the wavelength therebetween, the pair of light components oscillating in orthogonal directions. In general, a birefringent plate for use in a LCD device is implemented by a phase shifter made of a drawn film formed by drawing a high molecule polymer etc.
The xcex/4 plate 14 transforms the linearly polarized light into a circularly polarized light of clockwise rotation or counter-clockwise rotation. This allows the reflection LCD device to assume a bright state upon application of no-voltage and assume a dark state upon application of a voltage.
The reflection LCD device 100 is configured so that the angle xcex1 of the polarization absorption axis of the polarizing plate 15 resides between 5 and 35 degrees, the angle xcex2 of the optical axis of the xcex/2 phase shifter 14B resides between xe2x88x9215 and 15 degrees, and the angle xcex3 of the optical axis of the xcex/4 phase shifter 14A resides between xe2x88x9275 and xe2x88x9245 degrees. The angles recited herein are measured from the direction of the orientation of the front panel as a reference direction, with the rotational direction (twist direction) within the LC layer 30 as viewed from the front panel 10 to the rear panel 20 being positive.
By the configuration as recited above, the reflection LCD device 100 cancels altogether the change of the retardation which occurs depending on the visual angle of the LC layer and the change of the retardation which occurs depending on the visual angle of the phase shifters. Thus, the coloring on the display panel which occurs depending on the change of the observing azimuth can be cancelled, wherein the chroma on the display screen upon application of no-voltage is reduced down to below 20 in the color specification test for all the observing azimuths under a polar angle within xc2x160 degrees.
More specifically, the reflection LCD device solves the problems in that a yellowish coloring phenomenon which occurs depending on the observing azimuth causes displeasure to the user, and in that the coloring of the display which occurs at a higher visual angle causes displeasure to the user.
As described heretofore, the present inventor solved the problems of coloring defects on the display screen which occur depending on the observing azimuth and due to a higher visual angle in the reflection LCD device.
In a current cellular phone, a reflection LCD device mounted thereon for displaying a variety of functions is requested to have excellent image quality such as a higher contrast for assuring the user to easily read smaller characters in the display of the functions on the screen. In addition, the cellular phone is ever requested to have lower power dissipation and operate with a lower battery voltage for achieving a longer time service.
In the conventional reflection LCD device, although the coloring defects in the bright state caused depending on the observing azimuth and by a higher visual angle have been solved to some degree, there still remains another problem in that a lower driving voltage degrades the display contrast to lower the image quality of the reflection LCD device.
Although a specific technique, if employed, can solve the problem that the lower driving voltage degrades the display contrast in the conventional reflection LCD device, it raises the other problems that the yellowish coloring defect occurs depending on the observing azimuth and that an undesirable coloring defect occurs at a higher visual angle, both providing displeasure to the user.
In view of the above problems in the conventional reflection LCD devices, it is an object of the present invention to provide a reflection LCD device which is capable of solving the problems of the coloring defects occurring depending on the observing azimuth and at a higher visual angle, and also solving the problem of the lower driving voltage that degrades the display contrast.
The present invention provides a LCD device including a front panel, a rear panel and a LC layer sandwiched between the front panel and the rear panel, the front panel including: a front substrate having a front surface and a rear surface; a transparent electrode and a front orientation film consecutively formed on the rear surface of the front substrate; and a first phase shifter, a second phase shifter and a polarizing plate consecutively formed on the front surface of the front substrate, the rear panel including a rear substrate having a front surface and a rear surface, and a reflection electrode and a rear orientation film consecutively formed on the front surface of the rear substrate, the first phase shifter providing a retardation of 145 to 180 nm, the second phase shifter providing a retardation of 250 to 300 nm, angles of an optical axis of the first phase shifter, an optical axis of the second phase shifter and a polarization absorption axis of the polarizing plate with respect to a reference direction being xe2x88x9220 to 10 degrees, 40 to 70 degrees and 60 to 90 degrees, respectively, with a twist direction of the LC layer as viewed from the front panel to the rear panel being a positive direction, the reference direction being perpendicular to an orientation of the LC layer at a center of the LC layer between the front panel and the rear panel.
In accordance with the LCD device of the present invention, the first phase shifter has a retardation of 145 to 180 nm which is a sum of xc2xc (138 nm) of the central wavelength, or 550 nm, of the visible wavelength range and an additional retardation. The optical axis of the first phase shifter is arranged so that both the additional retardation and the residual retardation of the LC layer after application of a voltage cancel each other, whereby the reflection LCD device has an excellent dark state even at a lower applied voltage.
Thus, the reflection LCD device of the present invention solves the problem of the coloring defects occurring depending on the observing azimuth and due to a higher visual angle, and prevents reduction of the display contrast even at a lower driving voltage.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.